Some mobile telephone standards require simultaneous transmission and reception (what is known as full duplex). In this case, a duplexer typically reduces the transmission signal (TX signal) at the receiver input to a substantial degree. Nevertheless, there is a strong signal at the receiver input (RX input), and this signal is down converted to baseband in the reception mixer (RX mixer) and, upstream of the analog-to-digital converter (ADC), is attenuated, or in some cases even needs to be attenuated, by filtering. In other words, in some cases it is difficult to process a signal that has a comparatively weak level at a useful frequency and has a comparatively strong level at an undesirable interference frequency which is none too far from the useful frequency. A first pole (for example a first low-pass filter) downstream of the mixer performs current-to-voltage conversion using a first-order filter, that results in a large voltage swing at the output of the filter, since at the transmission offset frequency (Tx offset frequency) the first-order filter is able to achieve only a small amount of rejection.
The signal level of the transmission signal in the reception chain can be reduced by using intermediate stage filters in the RF chain, for example. Alternatively, it is possible to use a first pole having an operational amplifier, that can handle a large signal swing by using a large supply voltage. This can result in some systems not being able to be designed without an intermediate stage filter if the supply voltage would otherwise become too large.